The present invention relates to a semi-permeable membrane structure, and more specifically to one which can be used for separation operations in the medical field, particularly for plasmapheresis operations.
Plasmapheresis structures themselves are known, for example as disclosed in PCT Application No. 81/00,102. In the membrane structures disclosed in this PCT Application, the membrane is folded over onto itself, which makes it possible to dispense with the need to provide a seal in the region of the fold in the membrane. However, in the disclosed apparatus, it is necessary to provide two passage holes on one side of the folded membrane and to place tubular sealing devices around these holes to allow the blood to pass through.